There is a great need for amplifiers of reasonable cost and high reliability that are capable of amplifying extremely small signals to usable levels. This is particularly true for flow measuring devices, such as vortex-shedding flowmeters, where piezoelectric elements may be used as sensors for measuring pressure changes in fluid flow. U.S. Pat. No. 4,350,047, issued Sept. 21, 1982 to Clarence F. Dewey, Jr. and David E. Wiklund and assigned to Fisher Controls Company, Inc., discloses a vortex-shedding flowmeter. The copending U.S. patent application, Ser. No. 534,979, filed Sept. 23, 1983, of David E. Wiklund, Brian J. Burlage and Gary A. Lenz, assigned to Fisher Controls International, Inc., discloses a piezoelectric pressure sensor element. The charge generated by each of the above-noted piezoelectric pressure sensors in that flowmeter embodiment is approximately 2.times.10.sup.-12 coulombs at a flow rate of 4 Hz. Clearly, the need to amplify such a small, low frequency signal to a usable level places severe requirements on the charge amplifier design. This, of course, would usually result in an expensive unit or one that includes many design compromises.
Operational amplifiers which can perform this function are well known and are commonly available in a variety of configurations. Their cost, however, is commensurate with the stringency of their specifications. One very important characteristic of an operational amplifier intended for use with charge amplifiers is its "offset" voltage, which is essentially a low level signal inherent in the amplifier input. Unfortunately, any offset voltage is amplified along with input signals. Thus, if the amplifier is to be used with extremely low level input signals, the offset voltage of the amplifier becomes quite significant since it may have a magnitude near that of the input signal and may completely mask or obliterate the input signal. There is also an offset current which may flow in the inputs of the operational amplifier and which may be equally detrimental to its operation.
Another important factor in the design of charge amplifiers for flowmeter usage is that of the environment in which the flowmeters are situated. Flowmeters quite often are used in wet or humid environments and may even be completely exposed to the weather. Such environments are commonly considered to be "hostile" and very much unlike laboratory conditions which make for simpler designs. Such environmental conditions mandate that resistors of relatively low value be used. Typical resistors in charge amplifiers have values in excess of one megohm. Resistors of this magnitude will be very sensitive to foreign materials in the atmosphere or on the resistor surface, changes in temperature, the pressure and humidity of the environment and any deterioration in the resistor mounting structure, all of which will tend to increase leakage resistance and affect the resistor value.
The present invention solves the above shortcomings of the art with a circuit that is capable of using "off-the-shelf" type operational amplifiers, as opposed to those having specialized characteristics, such as extremely low offset voltages. Such off-the-shelf operational amplifiers may typically be purchased for less than one-tenth the cost of special operational amplifiers exhibiting extremely low offset voltage characteristics.